tag:blogger.com,1999:blog-5837660608809488753.post7844742799286298573..comments2024-03-28T20:53:49.167-04:00Comments on The Adventures of Roberta X: Hello, Hello, I'm Here -- Post-RoadtripRoberta Xhttp://www.blogger.com/profile/09956807794520627885noreply@blogger.comBlogger4125tag:blogger.com,1999:blog-5837660608809488753.post-5558175951859231962016-05-22T13:17:42.413-04:002016-05-22T13:17:42.413-04:00OPS: Alas, no circuit-diagram T-shirts. Plenty of...OPS: Alas, no circuit-diagram T-shirts. Plenty of other kinds for sale.<br /><br />RandyGC: well, rats! One day, eventually. I think I will try for the Ft. Wayne hamfest this year -- I have trouble getting used to having a dependable car. Tam always laughs at me when I pack a bag with a change of clothes, etc. for a day trip, just in case I get marooned.<br /><br />Anon: Ah, here's the thing, many MW broadcast towers are not a nice quarter or five-eights wavelength tall. They will often exhibit a complex impedance at the base that must be transformed with a matching network. But the time you've taken the whatever horrible umpty-jillion Ohms + (or -) j-zillion to a nice 50 + j0, you have interposed a tuned circuit of some kind between the antenna and the transmitter, and the transformation is only perfect at the carrier frequency. As a general, hand-wavy kind of rule, the greater the transformation, the farther off it will be away from resonance -- and right there is the very textbook definition of "bandwidth." Even in these dire days of an approximate 5 kHz upper limit for AM audio, you *want* the impedance to be flat across 10 kHz. In a bad situation, you may not get it.<br /><br /> And if that sounds unfortunate, remember in the old days you could set up an AM to have flat audio response from 20 Hz (if the modulator will take it!) to 20 kHz (if you can squeeze it through the tuned circuits, if anyone has that good an AM radio). A big, clumsily-designed matching network, or worse yet multi-tower phasing network, could have wretched R and j curves over the 40 kHz span need to pass that -- I remember one small town directional array in which both plots looked like sine waves! This tends to sound...bad. <br /><br /> (Note that in MW work, we always track Resistance (R) and reactance (j), or the "real" and "imaginary" components of impedance separately. The aim is to get j as low as possible, though many transmitters had slightly better performance with a slightly inductive or capacitive load. The hot app for this it an OIB, an "operating impedance bridge" that will take full power and has essentially zero insertion loss; you'd leave it in the circuit at all times with the R and j dials set for that the transmitter liked and use the "null" meter as a matching indicator. A directional array has a big gnarly phasing/power-dividing network anyway, so over the last 30 years or so, more and more stations added a couple of input-matching controls to it and built the OIB right in. Every slight change in the input impedance of the phasor + antenna array -- weather-related, or the water table rises, etc. -- shows up on the OIB null meter and you carefully tweak only the matching controls for the best null. The high power and wide bandwidth of MW broadcast stations tends to take things that are of little or no importance in a ham setup and turn them into real problems, of which all this is a prime example.)<br />Roberta Xhttps://www.blogger.com/profile/09956807794520627885noreply@blogger.comtag:blogger.com,1999:blog-5837660608809488753.post-64751364056923432732016-05-22T09:11:45.876-04:002016-05-22T09:11:45.876-04:00Looks like we missed each other again. I wasn'...Looks like we missed each other again. I wasn't sure if I was going to make it until Saturday morning. Didn't pick up any gear, but I got to touch bases with lots of friends from all over, which is more and more why I go each year.<br /><br />One of these times maybe we can to an eyeball QSL here or in Ft Wayne.<br /><br />73RandyGChttps://www.blogger.com/profile/16758726126424011542noreply@blogger.comtag:blogger.com,1999:blog-5837660608809488753.post-66934759818812300842016-05-22T04:33:21.893-04:002016-05-22T04:33:21.893-04:00I'm jealous of the antenna analyzer, it always...<br />I'm jealous of the antenna analyzer, it always struck me as a righteous way to visualize an antenna's impedance. <br /><br />I do have a broadcast engineering question for you, reading Paul (Thurst - spelling?) radio engineering's blog, he commented on one AM tower set-up seemed to be marginally acceptable for its bandwidth. Since the steel tower structure itself is the driven element, and circumference is usually at *least* 10-12 inches, if not more, with that 'fat' of a conductor, how in the hell is not broadbanded enough for a signal that's only about 15 khz wide?Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-5837660608809488753.post-571495579913554282016-05-21T15:28:51.848-04:002016-05-21T15:28:51.848-04:00Does the Hamvention still have somebody selling T-...Does the Hamvention still have somebody selling T-shirts with circuit diagrams on them? That was WAY back when I was fooling around on top band.Ole Phat Stuhttp://www.savory.de/blog.htmnoreply@blogger.com